The present invention relates to a method of driving a CCD sensor, and more particularly to the changing of a transfer pulse period with respect to a shift pulse period.
A CCD sensor, having a photosensitive section composed of a plurality of photosensitive picture elements, has been recently applied, as a light detecting element for converting optical images into electrical signals, to an image reading device such as an image scanner. In this case, generally all the picture element signals output by the picture elements in the photosensitive section are processed as image data. However, sometimes the picture element region at the middle of the photosensitive picture element array is a necessary picture element region having image data which must be output and the other two picture element regions on both sides of the necessary picture element region are accordingly unnecessary picture element regions having image data which need not be output. In this case, covers for blocking optical paths (namely, optically shielding boards) are provided over the two end portions of the photosensitive section in the CCD sensor, so that the picture element signals output by the two end portions are processed into predetermined signals. This keeps the unnecessary image data from being output.
The optical shielding board, having a shielding range determined with mechanical dimensional accuracy, should guard the necessary picture element region with an optical path nonblocking margin provided on the side of the unnecessary picture element region at its edge which is the border line between the necessary picture element region and the unnecessary picture element region. That is, because of the guard region, the entirety of the unwanted image data cannot be accurately kept from being output using the above-described optical shielding board.
Furthermore, although the effective image data is reduced, the unnecessary picture element regions output picture element signals having a predetermined level, and the total number of picture element signals is maintained as it is. Therefore, the signal processing time is not reduced thus impeding the high speed operation of the CCD sensor.